Communication systems typically include a plurality of dispatch consoles and communication units, such as mobile or portable radio units, that are geographically distributed among various base sites and console sites. The communication units wirelessly communicate with the base sites and each other, and are often logically divided into various talkgroups. Communication systems may be organized as trunked systems, where a plurality of radio frequency (RF) communication resources are allocated amongst multiple users or groups by assigning the base sites within a coverage area on a call-by-call basis, or as conventional (non-trunked) systems where RF communication resources are dedicated to one or more users or groups. In trunked systems, or in mixed trunked and conventional systems, there is usually provided a central controller/server (sometimes called a “zone controller”) for allocating RF communication resources among a group of sites. The zone controller may reside within a single device or multiple devices and may be located at a fixed equipment site or may be distributed among the base sites.
Calls between members of a talkgroup are typically accomplished by a user activating a microphone button and speaking into the microphone. Upon activation of the microphone button, the communication unit transmits a call request including a communication unit identifier and a talkgroup identifier to a zone controller. The zone controller identifies the talkgroup, locates other members of the talkgroup, and assigns an IP multicast group address for any participating sites (which may be base sites, console sites, or the like) to join. Once the participating sites are joined to the multicast group that is assigned to the talkgroup, the network distributes the call information (such as voice, data, video or the like) to participating sites that distribute the call information to the other members of the talkgroup as the user begins speaking.
While trunking systems work relatively well, they are adversely affected by long link delays between the various sites participating in the call. In particular, the participating sites may be located over multiple zones, and may be connected to the communication system via various transport types, such as T1s, E1s, Ethernet links, satellite links, etc. Because of the varying geographic distances and transport types, the call information distribution experiences varying delay characteristics. When participating sites having excessively long link delays are involved in the call, the call setup time (i.e., the time required for the participating site to be joined to the multicast group) may be too long and call information at the beginning of a transmission may be lost or truncated.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions and/or relative positioning of some of the elements in the figures may be exaggerated relative to other elements to help improve the understanding of various embodiments of the present invention. Also, common but well-understood elements that are useful or necessary in a commercially feasible embodiment are not often depicted in order to facilitate a less obstructed view of these various embodiments of the present invention. It will be further appreciated that certain actions and/or steps may be described or depicted in a particular order of occurrence while those skilled in the art will understand that such specificity with respect to sequence is not actually required. It will also be understood that the terms and expressions with respect to their corresponding respective areas of inquiry and study except where specific meaning have otherwise been set forth herein.